I currently have one in my car, and as a half cage it is legal, but with the front legs and intrusion bars bolted in it's not. Plus the car has now been rego'd as a two seater.
As for the belts there are harness that comply you just pay S@*t loads for them as they have to be mounted right as I found out.
There are many things that limit what you can do. There are ADR's on what you can have. Email me on shane.cross@syd.com.au and I will forward you the spec's I have for NSW.